The present application relates to apparatus for recovering the modulating signal from a frequency-modulated RF carrier signal and, more particularly, to a zero intermediate-frequency baseband signal processing demodulator.
It is well known to utilize a superheterodyne form of receiver, in which the received carrier signal is frequency converted to an intermediate frequency (IF) at which filtering and modulation detection are accomplished. While at least one IF amplifier-detector strip has been provided in integrated-circuit form (such as the LM373 IF integrated circuit available from National Semiconductor Co. and the like), a relatively large number of additional components, external to the integrated circuit, are required. Thus, the IF portion of a receiver cannot be provided solely in a monolithic integrated circuit.
The heterodyning of the received signal with a locally-generated signal at the carrier frequency, to produce a zero frequency, or baseband, IF signal for subsequent processing to detect the modulation thereon, is an idea well-known to the art. One such baseband IF demodulator, proposed by Vance in "An Integrated Circuit V.H.F. Radio Receiver", 50 The Radio and Electronic Engineer, No. 4, pp. 158-164 (April 1980) and in "Noise Considerations in an Integrated Circuit V.H.F. Radio Receiver", Proc. of the Conf. on Radio Receivers and Associated Systems, University of Leeds, pp. 49-66 (July 1981), utilizes differentiation of signals in crossed-multiplier arms and requires that the balance between the various arms be extremely well maintained to prevent distortion of the demodulated signal. The necessity to provide for balance adjustments mitigates against realization of this demodulator in a single monolithic integrated circuit. Accordingly, a baseband (zero IF) received signal demodulator capable of integration in a single IC, is highly desirable.